Have you ever Googled your own name? You might be surprised to see what information the internet holds about you. It's fun to relive career highlights and low-points from years ago, or find embarrassing shots of yourself posing for a team photo with the Sunday League football side you used to play for. The practice even has a cute name, "vanity searching".

But there's a flip side: if you can use the web to dig into your own history, then so can other people. And it's not just meddling friends or potential dates who may be tapping your name into search engines - employers are starting to use the trick, too. Suddenly, it doesn't seem so funny that a friend's MySpace page shows a photograph of you wearing a fluffy red wig and huge shades, flat-out drunk at the office Christmas party. And if you're trying to win a promotion at a law firm, you might come to regret every outré newsgroup post you made at university.

That's why online services are hoping to take care of the problem for you. ReputationDefender, a US-based company, has launched a service to monitor online information about anyone who signs up for their service. If it finds anything embarrassing, it will ask web hosts to take down the information - though it remains to be seen how many of them will comply. The site, which will also check to find out what your children are saying about themselves online, regularly searches the web and focuses on social networking, blogging, and photo-sharing sites.

But your reputation isn't the only thing that you need to protect online. From cyberstalkers to identity thieves, all kinds of people can find out all about you, and potentially wreak havoc with the results.

"The internet is a fantastic place for people to be, to buy, to shop - and to get information. But in today's society, your identity is the most precious thing you will ever have, and once you lose it, it could be lost forever," says Tony Neate, managing director of the Government initiative Get Safe Online, who also worked at the National High-Tech Crime Unit until its absorption into the Serious Organised Crime Agency earlier this year. "You have to take steps online to secure your identity."

We all know the dangers of identity theft - debts accrued in your name can wreak havoc, preventing you getting a mortgage, and your bank account can be targeted. But who knows what some people may do if they can build an extensive profile of you - it has never been so easy to acquire the information, so we can't know yet how it will be used.

How could someone go about snooping on you? Even if you have an ex-directory telephone number, the electoral roll will hold your details by default, unless you specifically choose not to be listed. "If you leave your information on it, your name, date of birth and home address are immediately available," warns Tom Ilube of Garlik, an identity protection service. Not only that, but the names of the people sharing the address will be available, too.

Garlik offers a privacy protection service called DataPatrol, which launched at the start of the month. It searches online for the personal information that could affect your privacy and scours data sources including credit reports, records at Companies House and the Land Registry, alongside more everyday search engines, monitoring for any changes.

Databases such as these can reveal extensive information about you to anyone who has your postcode. Websites such as nethouseprices.com and houseprices.co.uk will let people know what you paid for your house if they tap in your address. Correlate that against the electoral roll information about who lives there and you can begin building a rudimentary economic profile, one private investigator, who wishes to remain nameless, tells The Independent. "You can find out how much they earn because they will probably be sharing the mortgage," he says.

Local council planning permission databases can be another source of information. Some councils put scanned application forms online, displaying everything from detailed house plans to homeowner signatures. And if you have ever been a company director, a quick search of the Companies House database will yield your name, date of birth and address, along with other companies that you have directed, and your previous business partners.

Some services aggregate many of these online searches into one place - 192.com charges for such searches, gathering information from the electoral roll, the telephone database, and Companies House. It will even find information about a person's neighbours. Birth data online will often list the subject's mother's maiden name - a common security question asked by telephone banking services and other institutions.

Such information can be combined with postcode-searchable databases from the Office of National Statistics that provide extensive demographic information about the area surrounding a particular postcode. This provides online sleuths with useful data about everything from educational standards through to healthcare, economic deprivation and local crime statistics, not to mention census data. And so it continues. Every new piece of information can then be tapped into a search engine such as Google to reveal even more information.

Alongside the more traditional databases, social networking sites present a new hunting ground for personal data. To some, they are more than just a source of embarrassing pictures - they can also present a profound privacy risk, warns Simon Davies, director of the privacy advocacy group Privacy International. These sites, such as MySpace, its UK equivalent Bebo, and others such as Facebook, thrive on people's willingness to make connections with each other and post personal information.

"I think people on social networking sites sometimes suspend reality," Davies says. "They do things by impulse, they enter data unintentionally, they build friendships easily and without due care." Davies points to several examples of MySpace users posting pictures and information about themselves and their partners, along with friends posting birthday wishes - and therefore revealing birth dates, another common security question for phone-banking.

No wonder that privacy advisors like Privacy International's Davies and Garlik are urging people to be more careful online. So what can you do? The website of Get Safe Online has guidelines, and you could sign up for services from Garlik or ReputationDefender. But perhaps we should all start by taking more of an interest in what the internet knows about us - the websites (see box) will give you an insight, and there's nothing stopping you from asking companies, out of goodwill, to remove your details. After all, thanks to the internet, your life may well be an open book to those committed enough to read it.